ESTIMATION OF MONTHLY RAINFALL OVER OCEANS FROM TRUNCATED RAIN-RATE SAMPLES - APPLICATION TO SSM I DATA/

A physical-statistical monthly rainfall retrieval algorithm has been d eveloped using multichannel brightness temperatures from the Special S ensor Microwave/Imager (SSM/I). Since an emission-based retrieval algo rithm gives the most physically direct estimation of rainfall over oce ans, instantaneous rain rates are retrieved using brightness temperatu re-rain rate (T-R) relationships derived from a radiative transfer mod el. The retrieved rain rates, however, are only reliable and useful ov er a portion of a whole dynamic range of rain rate due to limitations of the emission-based algorithm. When monthly rainfall in a 5 degrees x 5 degrees box is estimated, the instantaneous rain-rate samples are actually truncated. The method used in this study assumes that monthly rainfall intensity in a 5 degrees x 5 degrees box has a mixed lognorm al distribution. Thus, the contribution of the rain rates outside of t he dynamic range can be estimated by extrapolation. Coefficients of th e mixed lognormal distribution are determined by fitting the truncated rain-rate samples to the lognormal form using a maximum likelihood es timate method. The beamfilling error is corrected by a multiplicative factor generated from simulation studies. Comparison between the month ly rainfall estimated from the SSM/I and Pacific atoll data indicates that the algorithm works very well in tropical areas. Although this al gorithm is tested on SSM/I data, it is also suited for the Tropical Ra infall Measuring Mission data, which should have a larger dynamic rang e with 10.7-GHz channels added.